Organ in insects and biased chirality

In many insects the male copulatory or the intromittent organ or the aedeagus undergoes a rotation around its longitudinal axis through 180 degrees (in some flies or Diptera through 360 degrees) during development. It is believed that this change of orientation of the organ facilitates a particular mode of copulation, the riding mode, in which the male climbs over the back of the female, and then brings about intromission (Jeannel, 1955). The riding mode is believed to be the more advanced mode, and end-to-end copulation, in which the male and the female, resting on the same surface, face opposite directions, and bring their abdominal ends together for intromission, is believed to have preceded the riding mode in insect evolution.

There is an important difference between the rotation of the aedeagus in Coleoptera and that in Diptera. In beetles the rotation is due to asymmetric development of a pair of corresponding muscles, connected with the aedeagal apparatus. These muscles arise from the ventral body wall (or a tubular invaginated part of it), and find attachment on the dorsal face of the developing copulatory tube. During development one of the muscles degenerates, while the surviving one exerts a one sided pull, making the copulatory organ rotate through 180 degrees. In Diptera rotation of the copulatory apparatus is due to some terminal abdominal segments rotating through varying degree on preceding segments, so that the enclosed aedeagal apparatus is made to rotate through 180 or 360 degrees. In beetles rotation of the aedeagus occurs without affecting the original symmetry of terminal abdominal segments.

Among Coleoptera the aedeagal rotation is clockwise, when seen from behind, and only rarely anticlockwise. Interestingly in Diptera too the rotation is mostly clockwise, though the mechanism of rotation is very different from that in beetles. There is enough reason to believe that rotation of the aedeagus has evolved independently in Coleoptera and Diptera, but clear bias for clockwise rotation is found in both the insect groups.

If an object or a phenomenon is identical with its mirror image, it is called chiral, or it is described as showing chirality. The symmetry of human body is chiral, as in our mirror image the right hand appears left. It has been realized that most objects and phenomena in the universe show chirality (Hegstrom et al, 1990). One surprising situation with chirality is that generally it is associated with bias in favour of one variant or the other. Human individuals may be right handed or left handed, but lefties are comparatively rare, though a left handed person does not have any intrinsic disadvantage. In this instance, there is an obvious bias in favour of the right handedness. Gastropods present clockwise coiling in the shell, if we look at the shell with its apex towards us; such shells are called dextral. Some shells may be sinistral, as they show anticlockwise coiling. In any gastropod species dextral shells are much more common than sinistral ones. Thus in this case chirality shows bias in favour of dextrality. Most plants, which helically coil round a support, for example Convolvulus arvensis, present generally a right handed spiral. A recent example is found with the study of New Caledonian crows, the ones which manufacture tools from Pandanus leaves (Corvus moneduloides). Weir et al. (2004) studied laterality of tool use in captive birds: five were left lateralized and five were right-lateralized. All subjects show near-exclusive individual laterality. The predominance of right-handedness in humans is not general, but all, including non-human primates, show strong individul laterization for tool use.

Bias in chirality is not confined to anatomy; it is even at the molecular and atomic levels. Amino acids, which polymerize to form proteins, present chirality in their molecular structure, but for glycine. They may be in dextro- or levo- form (D- or L-), which rotate the plane of polarization of a polarized beam of light in clockwise and anticlockwise directions respectively. Surprisingly, in natural proteins all amino acids are with the L-form; only very rarely D-amino acid molecules may be included. DNA and RNA molecules almost always present right handed helixes. Beta rays, produced in radioactive decay, also show biased chirality; they include mostly left handed electrons.

It is not understood whether and how biased chirality at the atomic or molecular level is related to lack of parity in chirality in anatomy.

As has been pointed out earlier, rotation of the copulatory organ in insects is a phenomenon with biased chirality, bias in favour of clockwise rotation. About the mechanism of this phenomenon, it has been mentioned that in beetles the rotation occurs due to one member of a pair remnants of the degenerating left protractor muscle

¿.[•^•Ar ~ or the aedeagus genetal pocket enclosing the developing aedeagus right protractor muscle an invagination of intertegument

— Fig. 31.1. Developing male genitalia of a leaf-beetle. The genitalia have been cut across, and have been looked at from behind.

of corresponding muscles degenerating, and the surviving muscle making the organ rotate. Usually it is the left member of the pair that degenerates; as a result the rotation is clockwise in most cases. The two muscles in the pair are very similar; they are laterally corresponding and homologous. Then how is it that generally it is the left member of the pair which undergoes degenration? One of us (KKV), with his coworkers, through experimentation, have inferred that, if there is normal release of the juvenile hormone (JH) after the last moult, it is followed by degeneration of the left muscle, and consequent clockwise rotation of the copulatory organ, while a delayed release of the hormone seemingly leads to degeneration of the right muscle and anticlockwise rotation. The two muscles in the pair are identical in appearance. Why are they differently affected by the time of release of JH after the imaginal moult? Are the two muscles differently programmed for cell death? Is the biased chirality at the atomic/molecular level in some way responsible for this differential programming? These questions remain wholly unanswered, and the biased chirality in rotation of the male copulatory organ remains an unsolved puzzle.